1. Field of the Invention
This invention relates to the bonding of poly(vinylidene fluoride) to a solid substrate, and more particularly to the surface treatment of the poly(vinylidene fluoride) and the substrate and the use of primers and coupling agents to effect the bond.
2. Description of the Prior Art
Poly(vinylidene fluoride) is a polymer which has been found to exhibit "piezoelectric" properties when suitably processed. The term "piezoelectric" is used herein to denote the property of a material by which the application of a mechanical stress to the material produces a potential across the material. The term also connotes the converse property (which is also present to some degree) by which application of a potential through the material produces a stress and resultant mechanical strain. The poly(vinylidene fluoride) material has been found to exhibit the piezoelectric property only after non-random or crystalline molecular orientations have been created, and after polarization has occurred in respect to these oriented molecular constituents. The orientation takes place at room temperature and above, and may be destroyed if care is not exerted in subsequent treatment of the material. Exposure to excessive electric fields or excessive temperature may have this effect. Given that the material has suitable piezoelectric properties for proper use, it is essential that means be found to support it for exposure to the pressure that is to be responded to and for electrical contact to derive the resultant output potential. One such application of this piezoelectric material is to the sensing of acoustic vibrations in sea water, typified by a hydrophone.
An object in such applications is then one of providing a suitable substrate for the poly(vinylidene fluoride) and bonding the two together with adequate bond strength and good electrical contact.
Accordingly, it is an object of the invention to provide a novel method of bonding poly(vinylidene fluoride) to a substrate.
It is another object of the invention to provide a novel method of bonding poly(vinylidene fluoride) to a substrate with improved bond strength.
It is still another object of the invention to provide a novel method of bonding poly(vinylidene fluoride) to a substrate in a piezoelectric application with improved bond strength and facilitating good piezoelectric properties.
These and other objects of the invention will be dealt with in the description which follows. They are achieved in a novel method which includes preparing a poly(vinylidene fluoride) (PVF.sub.2 ) solid for bonding to a solid substrate comprising the steps of abrading the surface to provide an irregular bonding surface, rinsing the surface to remove surface contaminants; plasma etching the surface to create a temporary high energy bonding surface by activated free radical and ionic species of oxygen, while keeping the PVF.sub.2 solid at a temperature below the melting point of its crystallites for dimensional stability or below the temperature for reversion to the unpolarized state; and applying a primer to the surface of the PVF.sub.2 solid capable of wetting the activated high energy surface of the PVF.sub.2, before the temporary high energy bonding surface has deteriorated. The primed surface of the PVF.sub.2 is then ready for adhesive bonding to a suitably prepared substrate. Suitable substrates are steel (including stainless steel), brass, copper and aluminum as well as a variety of structural organic compositions. The substrate preparation involves similar steps including abrading the surface, rinsing the surface, and dependent on the substrate, the application of a suitable coupling agent.
The inventive method is also applicable to the case where the adhesive is applied directly to the treated surface of the poly(vinylidene fluoride). In this case, the application of the adhesive must occur before the treated surface has deteriorated, and the adhesive selected must be one that will wet both the treated surface of the poly(vinylidene fluoride) and prepared surface of the substrate.